Glyphosate

Glyphosate, N-phosphonomethyl glycine, is broad-spectrum herbicide, sold under the brand name RoundUp. It is "the most widely used broad-spectrum herbicide on [a] global scale." Commercial formulations of glyphosate were first sold in 1974. Some genetically engineered crops, called Roundup Ready Crops, have their DNA altered to allow them to withstand glyphosate. These include Roundup Ready soybeans, corn, canola, sugarbeets, and alfalfa.

How It Works
Glyphosate is absorbed through a plant's foliage and then transported throughout the stems, leaves, and roots of the entire plant.


 * "Glyphosate inhibits plant growth by inhibiting the production of essential aromatic amino acids through competitive inhibition of the enzyme enolpyruvylshikimate phosphate (EPSP) synthase. This is a key enzyme in the shikimic acid pathway for the synthesis of chorismate..., which is a precursor for the essential amino acids phenylalanine, tyrosine, and tryptophan."

In other words, glyphosate prevents plants from making amino acids they need to survive. It does this by inhibiting an enzyme needed to make chorismate, a precursor to those amino acids. One of these amino acids, tryptophan, "is necessary for the synthesis of indolylacetic acid (IAA), the main growth promoter, that can explain the widespread ﬁeld observation of reduced in depth root growth of plants."

A 1984 study found plants that died following treatment with glyphosate were infected with pathogenic fungi, compared to control plants not treated with glyphosate but planted in the same media that did not yield pathogenic fungi. The study concluded that more research was needed but postulated that glyphosate inhibits the plant's defense mechanisms and/or increases nutrient leakage from treated plants.

When more research was completed, "Rahe and coworkers documented that severe root infection associated with glyphosate-treated plants was due to disruption of synthesis of plant defense compounds, or phytoalexins, through the shikimate pathway there by predisposing plants to attack by soilborne fungal pathogens (Johal and Rahe, 1988; Lévesque et al., 1987). Thus, infection by soilborne pathogens caused by the inability of plants to synthesize phytoalexins contributed to the overall herbicidal efficacy of glyphosate and was considered a “secondary mode of action” of glyphosate. These ﬁndings were significant because the release of glyphosate into the environment was found to have considerably more and far-reaching effects than the original notion that was limited to only the localized disruption of a specific metabolic pathway within a target plant."

Impact on Non-Target Plants
Once glyphosate travels to a plant's roots, it is "released into the rhizosphere," (the area immediately around the roots), "where it is immobilized at the soil matrix or microbially degraded. However, some of the glyphosate remains in dead plant tissues. A 2009 study found that non-target plants continue to be impacted by glyphosate toxicity up to three weeks after glyphosate application.

A 2009 study examined the relationship between previous glyphosate use, tillage method (conventional, minimal, or no-till), and plant diseases caused by fungal pathogens of the genus Fusarium. The study found "a relationship between previous glyphosate use and increased Fusarium infection of spikes and subcrown internodes of wheat and barley, or Fusarium colonization of crop residues." However, the study adds, "because of the close association between noncereal crops, reduced tillage, and glyphosate use, it was not possible to completely separate the effects of these factors on Fusarium infections." (No-till or minimum till practices are often combined with herbicide use, specifically, glyphosate use. Thus, the scientists were not able to determine whether any increase or decrease in plant disease was related to tillage strategy, glyphosate use, or both.) The study recommended more research in this area.

Impacts of Glyphosate Drift
At sub-lethal doses of glyphosate, such as the amounts a plant might be exposed to from spray drift, plants are still impacted. A study examined sunflowers treated with small amounts of glyphosate (to simulate spray drift) found:
 * "In conclusion, the results presented in this study showing that glyphosate is especially inhibitory to ferric reductase complement the recently published report (Eker etal., 2006) that glyphosate exerts a strong inhibitory influence on ferric reductase activity of Fe-deficient roots and impairs the uptake and translocation of Fe in plants. These impairments could be a major reason for the increasingly observed Fe deficiency chlorosis in cropping systems associated with widespread glyphosate usage as reported for different parts of the USA (Franzen etal., 2003; Jolley etal., 2004). Such strong interference of glyphosate with root uptake and root-to-shoot transport of Fe in crop plants may represent a potential threat to human and animal nutrition because of possible reduction of Fe in edible plants parts (e.g. seed/grain)."

In other words, low doses of glyphosate equal to the amount plants are exposed to in spray drift, can result in iron deficiencies in the plants. For crops destined as animal feed or as human food, this could result in decreased dietary iron.

Impact on Microorganisms
Several sources have found that glyphosate impacts soil microorganisms. For example:


 * "Subsequent research on glyphosate interactions with soil microorganisms demonstrated that although glyphosate was metabolized by a segment of the microbial population, it was also toxic to several bacteria and fungi; the net effect glyphosate appeared to be a disruption of soil and root microbial community composition because selected components of the microbial community were stimulated while others were suppressed."


 * "Herbicides, including glyphosate, can inhibit or stimulate the growth of fungal pathogens, and can either increase or decrease disease development through direct or indirect means (Altman, 1993; Levesque and Rahe, 1992). Levesque and Rahe (1992) showed evidence that herbicides can have a direct effect on various components of the soil microﬂora, such as plant pathogens, antagonists, or mycorrhizae, which can potentially increase or decrease the incidence of plant disease. Pathogens able to infect weeds can also increase their inoculum potential after weeds have been sprayed with herbicides, which could subsequently affect host crops."

Glyphosate in the Environment
Although glyphosate is highly soluble in water, its tendency to bond to soils makes it unlikely to leach into groundwater or runoff "significantly." (Studies have found about 1%-2% of glyphosate may runoff in rainfall after glyphosate is applied. )

Glyphosate in the Soil
Glyphosate can reach the soil by washing off the foliage of plants, via spray drift, by exudation from the roots of treated plants, or by the decomposition of treated plants. However, "risks of glyphosate toxicity to non-target organisms in soils are generally considered as marginal,since glyphosate is almost instantaneously inactivated by adsorption to clay minerals and cationic binding sites of the soil matrix (Piccoloetal.,1992;Dong-Meietal.,2004), while glyphosate in the soil solution is prone to rapid microbial degradation (Giesy et al., 2000)." In other words, glyphosate residues in the soil are not considered hazardous as it either breaks down quickly or binds to minerals that make it no longer a threat to plants. Glyphosate that biodegrades usually breaks down into carbon dioxide and ammonium (NH4+). In an analysis of 47 studies, 50% of glyphosate broke down in the soil in time periods ranging from 1.2 days to 197.3 days. The arithmetic mean amount of time was 32 days and the geometric mean was 17 days.

Although glyphosate is mostly broken down by microbes or bound to the soil, a 2009 study found that "the root tissue of glyphosate-treated weeds represents a storage pool for glyphosate."

Glyphosate in Water
Although most glyphosate applied to soil does not run off into waterways, sometimes glyphosate is applied to aquatic environments directly. In flowing water, it is dissipated via "tributary dilution, dispersion, and loss through processes such as absorption to suspended particulate matter or sediments and microbial degradation." The half-life of glyphosate in water has been estimated to be from 7 to 14 days.

Related SourceWatch articles

 * Monsanto
 * Roundup Ready Crops
 * Biotechnology

External resources

 * Bott, S., Tesfamariam, T., Kania, A., Eman, B., Aslan, N., Roemheld, V., and Neumann, G. 2011, Phytotoxicity of glyphosate soil residues re-mobilised by phosphate fertilization. Plant Soil 315:2-11. DOI 10, 1007/s11104-010-06989-3.
 * Schafer, J.R., Hallett, S.G., and Johnson, W.G. 2010. Role of soil-borne fungi in the response of giant ragweed (Ambrosia trifida) biotypes to glyphosate. Proc. Northcentral Weed Sci. Soc. 65:.
 * Zobiole, L.H.S., Oliveira, R.S.Jr., Huber, D.M., Constantin, J., Castro, C., Oliveira, F.A., Oliveira, A. Jr. 2010. Glyphosate reduces shoot concentrations of mineral nutrients in glyphosate-resistant soybeans. Plant Soil 328:57-69.
 * Zobiole, L.H.S., Oliveira, R.S. Jr., Kremer, R.J., Constantin, J., Yamada, T., Castro, C., Oliveiro, F.A., and Oliveira, A. Jr. 2010. Effect of glyposate on symbiotic N2 fixation and nickel concentration in glyphosate-resistant soybeans. Applied Soil Ecol. 44:176-180.
 * Bellaloui, N., reddy, K.N., Zablotowicz, R.M., Abbas, H.K., and Abel, C.A. 2009. Effects of glyphosate application on seed iron and root ferric (III) reductase in soybean cultivars. J. Agric. Food Chem. 57:9569-9574.
 * Cakmak, I., Yazici, A., Tutus, Y., Ozturk, L. 2009. Glyphosate reduced seed and leaf concentrations of calcium, magnesium, manganese, and iron in non-glyphosate resistant soybean. European J. Agron. 31:114-119.
 * Kremer, R.J. and Means, N.E. 2009. Glyphosate and glyphosate-resistant crop interactions with rhizosphere microorganisms. European J. Agron. 31:153-161.
 * Johal, G.R. and Huber, D.M. 2009. Glyphosate effects on diseases of plants. European J. Agron. 31:144-152.
 * M.R. Fernandez, R.P. Zentner, P. Basnyat, D. Gehl, F. Selles, and Don Huber, "Glyphosate associations with cereal diseases caused by Fusarium spp. in the Canadian Prairies," European Journal of Agronomy (2009), 31:133-143.
 * Yamada, T., Kremer, R.J., Camargo e Castro, P.R., and Wood, B.W. 2009. Glyphosate interactions with physiology, nutrition, and diseases of plants: Threat to agricultural sustainability? European J. Agron. 31:111-113.
 * Schafer, J.R., Westhoven, A.M., Kruger, G.R., Davis, V.M., Hallett, S.G., and Johnson, W.G. 2009. Effect of growth media on common lambsquarter and giant ragweed biotypes response to glyphosate. Proc. Northcentral Weed Sci. Soc. 64:102.
 * Tsehaye Tesfamariam, S. Bott, I. Cakmak, V. Römheld, G. Neumann, "Glyphosate in the rhizosphere – role of waiting times and different glyphosate binding forms in soils for phytoxicity to non-target plants," ''European Journal of Agronomy (2009), 31:126-132.
 * Levent Ozturk, Atilla Yazici, Selim Eker, Ozgur Gokmen, Volker Römheld, and Ismail Cakmak, "Glyphosate inhibition of ferric reductase activity in iron deficient sunflower roots," New Phytologist (2008), 177:899-906.
 * Eker, S., Ozturk, L., Yazici, A., Erenoglu, B., Roemheld, V., and Cakmak, I. 2006. Foliar-applied glyphosate substantially reduced uptake and transport of iron and manganese in sunflower (Helianthus annuus L.) plants. J. Agric. Food Chem. 54:100019-10025.
 * Larsen, R.L., Hill, A.L., Fenwick, A., Kniss, A.R., Hanson, L.E., and Miller, S.D. 2006. Influence of glyphosate on Rhizoctonia and Fusarium root rot in sugar beet. Pest Manag. Sci. 62:1182-1192.
 * Johal, G.R. and Rahe, J.E. 1990. Role of phytoalexins in the suppression of resistance of Phaseolus vulgaris to Colletotrichum lindemuthianum by glyphosate. Canad. J. Plant Pathol. 12:225-235.
 * Gurmukh S. Johal and James E. Rahe, "Effect of soilborne plant-pathogenic fungi on the herbicidal action of glyphosate on bean seedlings," Phytopathology (1984), 74:950-955.

External articles

 * Jill Richardson, "Why Is Damning New Evidence About Monsanto's Most Widely Used Herbicide Being Silenced?" Alternet, April 27, 2011.
 * William Neuman and Andrew Pollack, "Farmers Cope With Roundup-Resistant Weeds," New York Times, May 3, 2010.